It's quite possible to do an impressive water simulation in real time. Here's one on WebGL.[1] Very impressive demo. But you need most of a GPU to do it.

The fountains in GTA V are just playing a short texture animation on repeat, plus a particle effect for water droplets. They have a whole game to run and can't devote too many resources to water. GTA V does amazingly well at keeping a large number of active objects moving around.

GTA's ocean water is quite elaborate. There are waves, you can jet-ski on it, and the effects all work together well. Much more impressive than the fountains.

I once briefly met the guy who did the first good water shader, for Softimage. That had synchronized levels of detail, so you could see fine detail close up yet see all the way to the horizon, and the waves matched up. Not real-time, but it was the 1990s. First used on Waterworld, which didn't get that guy any attention. Then used on Titanic, which did.

[1] http://madebyevan.com/webgl-water/